The dissemination of traffic information to motorists is usually carried out by transmitters operating in the UHF band whose carrier waves can be pulled in only with a rather precise adjustment of the tuning circuit of a receiver. Drivers required to pay attention to road traffic cannot concentrate for long periods on the tuning mechanism of their receivers whose adjustment by the usual knobs tends to pass rather quickly through the desired UHF station. It has therefore become customary to modulate the carriers of such transmitters with a pilot tone above the audible-frequency range, e.g. of 57 kHz, which can be detected by suitably designed receiving equipment.
In a known receiver of this type, a switch is provided for disconnecting the audio stage from the preceding r-f (or i-f) stage whenever it is desired to pick up a traffic-monitoring station characterized by the aforementioned pilot tone. Upon manipulation of the tuner, a detector in the receiver recognizes the pilot frequency and reconnects the audio stage to enable the reception of messages. When the incoming high-frequency signal fades, the detector again disconnects the audio stage from the remainder of the system.
Thus, a driver passing through zones of poor reception is bound to experience an annoying alternation of cutoff and cut-in of the station to which the receiver is tuned.